The present invention relates in general to an electrophotographic imaging device, and more particularly to systems and methods for shifting the dynamic range of laser power of a laser beam, e.g., for discharging a photoconductive surface using a laser beam that is also used for writing image data during imaging operations.
In electrophotography, an imaging system forms a latent image by exposing select portions of an electrostatically charged photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to the laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner pattern is subsequently transferred from the photoconductive surface to the surface of a print substrate, such as paper, which has been given an electrostatic charge opposite that of the toner.
A fuser assembly then applies heat and pressure to the toned substrate before the substrate is discharged from the apparatus. The applied heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the applied pressure promotes settling of the toner constituents in these voids. The toner solidifies as it cools adhering the image to the substrate.
During operation of the electrophotographic device, if a charge roll of the imaging system is turned off and the associated photoconductive surface carries an excessive electrostatic charge, there is the potential for print artifacts such as ghost images, color shifts and other residual image artifacts on the first page of the first print job after restarting the device. However, print artifacts that may occur as a result of transiently turning on and off the imaging system can be mitigated by discharging the photoconductive surface to a generally consistent, intermediate level by implementing a run out process as part of a power down sequence of operations.
In conventional printing systems, discharge operations are performed using an erase assembly. The erase assembly typically includes a light source, such as a fluorescent tube or Light Emitting Diode (LED) array, which is positioned at each transfer station so as to face the image area of a corresponding photoconductive surface. Alternatively, light emitted by the light source may penetrate a semi-transparent layer, e.g., by positioning the erase assembly on a side of an intermediate transfer belt (ITM belt) opposite from the photoconductive surface, e.g., a photoconductive drum (PC drum). In this configuration, light from the light source shines through the ITM belt and partially discharges the PC drum during the run out process. Regardless of which conventional architecture is used, the erase assembly requires a light source positioned about the photoconductive surface, which affects the size of the imaging system.